Hi,
Can a past tense work, as well as the one used, the present perfect, here -- when we have a phrase 'for the last three weeks' or a similiar one thereof?
I have written my friend a letter every other day for the last three weeks.
I wrote my friend a letter every other day for the last three weeks.
I wouldn't say it's wrong, although it sounds a little strange to me. However, by dropping the Preent Perfect, you are dropping the meaning of 'this has some importance to the prsent situation'. In other words, you are changing the meaning.
Best wishes, Clive